The present invention is directed to a system and method of measuring the surface properties of chemical mechanical polishing pads used for creating a smooth, ultra-flat surface on such items as glass, semiconductors, dielectric/metal composites, magnetic mass storage media and integrated circuits. More specifically, the invention is directed to the use of noncontact reflectance ultrasound to measure the surface coating and wearing patterns of polishing pads.
Chemical-mechanical polishing (CMP) is used extensively as a planarizing technique in the manufacture of Very Large-Scale Integration (VLSI) integrated circuits. It has potential for planarizing a variety of materials in IC processing, but is used most widely for planarizing metallizied layers and interlevel dielectrics on semiconductor wafers, and for planarizing substrates for shallow trench isolation.
The growing use of copper for circuit interconnects, but lack of etching techniques to remove copper, has led to the adoption of damascene processes and the use of CMP to remove excess copper and associated barrier metals. In shallow trench isolation, for example, large areas of field oxide must be polished via to produce a planar starting wafer. Achieving acceptable planarization across the full diameter of a wafer using traditional etching processes has been largely unsuccessful. However, using conventional CMP, where the wafer is polished using a mechanical polishing wheel and a slurry of chemical etchant, unwanted oxide material is removed with a high degree of planarity.
Similarly, multilevel metallization processes, each level in the multilevel structure contributes to irregular topography. Planarizing interlevel dielectric layers, as the process proceeds, is often now favored in many state-of-the-art IC fabrication processes. High levels of planarity in the metal layers is a common objective, and this is promoted by using plug interlevel connections. A preferred approach to plug formation is to blanket deposit a thick metal layer, comprising, for example W, Ti, TiN, on the interlevel dielectric and into interlevel windows, and then removing the excess metal using CMP. CMP may also be used for polishing an oxide layers, such as SiO2, Ta2O5 or W2O5 or to polish nitride layers such as Si3 N4, TaN, TiN.
There are, however, deficiencies in our understanding the multiple factors that affect CMP performance. These deficiencies derive in part from the lack to nondestructive methods to evaluate efficacy of steps in the production CMP pads, as well as evaluating the wearing characteristics of such pads. For example, the mechanical and chemical properties of CMP pads may be evaluated by dynamic mechanical analysis (DMA) and Fourier Transform Infrared (FTIR) Spectroscopy, respectively. Such measurements, however are performed on strips or samples of material cut from pads. These approaches, therefore, are not ideally suited to provide information about the dynamics of pad production and wear during use. Ultrasound provides a potential means to nondestructively evaluate these properties.
Noncontact optoacoustic metrology, for example, using laser light to generate and detect ultrasonic waves, has been used to characterize metal deposition and uniformity on semiconductor wafers before and after CMP. Techniques that bounce an acoustic signal off of the wafer being polished, similar to sonar principles, have been used to detect polishing end points. Direct transmission scanning ultrasound, where an ultrasonic signal generated by a transducer attached to a pad is passed through the pad to a receiver on the opposite side of the pad, has been used to detect inhomogeneities in ultrasonic transmission amplitudes, possibly related to the pad""s density, elastic modulus or viscosity coefficient. This approach, however, requires intimate contact between the measuring device and the material being tested, either by immersing the material in a coupling fluid or by vacuum suction of a sensor to the material""s surface. Such contact disturbs the surface that is being measured. Such measurement approaches are also un acceptably slow, for example, requiring more than one day to measure the surface of a polishing pad. Moreover, none of the above described approaches, address inspecting of pad surfaces during their production and monitoring the pad""s wear characteristics.
Accordingly, what is needed is an improved method of using ultrasound to nondestructively monitor the production and wearing patterns of the surface of CMP pads, while not experiencing the above-mentioned problems.
To address the above-discussed deficiencies, the present invention provides, in one embodiment, a system for measuring surface properties of a polishing pad. The system comprises a polishing pad having a polishing surface associated therewith, an ultrasonic probe located over the polishing surface and a subsystem coupled to the ultrasonic probe. The probe is configured to both transmit an ultrasonic signal to the polishing surface and receive a modified ultrasonic signal from the polishing surface without contacting the polishing surface. The subsystem is configured to determine a surface property of the polishing pad from the reflection.
In yet another embodiment, the present invention provides a method for measuring the surface properties of a polishing pad. The method includes situating an ultrasonic probe above a polishing surface of a polishing pad, without contacting the polishing surface. The method further comprises transmitting an ultrasonic signal from the probe to the polishing surface, the ultrasonic signal being modified by the polishing surface. The method also includes receiving the modified signal by the ultrasonic probe.
The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the scope of the invention.